Cellulose food casings are well known in the art and are widely used in the production of stuffed food products such as sausages and the like. These generally are seamless tubes formed of a regenerated cellulose and contain a plasticizer such as water and/or a polyol such as glycerine. Plasticization is necessary because otherwise the cellulose tube is too brittle for handling and commercial use. A non reinforced cellulose for use as a food casing is a tubular film of various sizes having a wall thickness ranging from about 0.025 mm to about 0.076 mm and made in tube diameters of about 14.5 mm to 203.2 mm.
The cellulose for making this casing is most commonly produced by the well known "viscose process". Briefly, in the viscose process a natural cellulose such as wood pulp or cotton linters first is treated with a caustic solution to activate the cellulose to permit derivatization and extract certain alkali soluble fractions from the natural cellulose. The resulting alkali cellulose is shredded, aged and treated with carbon disulfide to form cellulose xanthate. The cellulose xanthate is dissolved in a weak caustic solution. The resulting solution or "viscose" is ripened, filtered, deaerated and extruded.
For food casing, the viscose is extruded as a tube through an annular die and about a self centering mandrel into coagulation and regenerating baths containing salts and sulfuric acid. In the acidic baths the cellulose xanthate, e.g., viscose, is converted back to cellulose. The acid bath decomposes the cellulose xanthate in a chemical reaction with the result that a pure form of cellulose is coagulated and regenerated. Initially, the coagulated and regenerated cellulose is in a gel state. In this gel state the cellulose tube first is run through a series of rinse water dip tanks to remove by-products formed during regeneration. The gel tube then is treated with a glycerine humectant and dried to about 10% moisture based on total casing weight. As noted above, the gel tube is inflated during the drying process to a pressure sufficient to provide a degree of orientation to the dried cellulose tube.
Despite certain well-known problems inherent with the viscose process including the production of noxious products during regeneration of the viscose, it nevertheless remains today as the most commonly used process for the production of cellulose casing for the food processing industry.
An alternate cellulose production method involves forming a cellulose solution by means of a simple dissolution rather than requiring prior derivatization to form a soluble substance (as in the viscose process). U.S. Pat. No. 2,179,181 discloses the dissolution of natural cellulose by a tertiary amine N-oxide to produce solutions of relatively low solids content, for example 7 to 10% by weight cellulose dissolved in 93 to 90% by weight of the tertiary amine N-oxide. U.S. Pat. No. 3,447,939 discloses use of N-methyl-morpholine-N-oxide (NMMO) as the tertiary amine N-oxide solvent wherein the resulting solutions, while having a low solids content, nevertheless can be used in chemical reactions involving the dissolved compound, or to precipitate the cellulose to form a film or filament.
More recent patents such as U.S. Pat. Nos. 4,145,532 and 4,426,288 improve upon the teachings of the '939 Patent. U.S. Pat. No. 4,145,532 discloses a process for making a solution of cellulose in a tertiary amine oxide such as NMMO which contains 10-35% by weight of cellulose. This higher solids content, achieved in part by including an amount of water (from 1.4% to about 29% by weight) in the tertiary amine oxide solvent, provides a solution adapted for shaping into a cellulosic article such as by extrusion or spinning. In U.S. Pat. No. 4,426,288 the NMMO-cellulose solution contains an additive which reduces decomposition of the cellulose polymer chain so that molding or spinning substances are obtained with only slight discoloration and which will yield molded shapes distinguished by improved strengths upon precipitation in a nonsolvent such as water.
Using NMMO as a solvent for cellulose eliminates the need for derivatizing the cellulose, as in the viscose process. Consequently, it eliminates the disadvantages attendant to the viscose process such as the problems associated with the generation of toxic and noxious products such as gases and sulfur compounds during regeneration of the viscose.
Even with these advantages, to applicants' knowledge, NMMO-cellulose solutions heretofore have been used commercially, primarily to manufacture fibers and filaments and not in the commercial manufacture of cellulose food casings. This perhaps is due in part to the fact that the nonderivatized cellulose solution is thermoplastic with a melting point of about 65.degree. C. so it is normally solid at the temperature heretofore used in the extrusion of viscose (e.g. cellulose xanthate) for producing cellulose food casings.
It is speculated that another reason why nonderivatized cellulose has not been commercially used in manufacture of food casings is that the solution at 65.degree. C. has a viscosity significantly higher than the viscosity of the derivatized cellulose heretofore used in the production of cellulose food casings. In particular, nonderivatized cellulose in solution may have a molecular weight of about 80,000 to 150,000 and a viscosity in the range of about 1,000,000 to 3,500,000 centipoises. The high molecular weight and viscosity is because the dissolution of the cellulose does not affect the degree of polymerization. Viscose for casing manufacture (wherein the degree of polymerization is affected by the derivatization process) has a molecular weight in the range of about 95,000 to 115,000 for nonfibrous casing and a viscosity of 5,000 to 30,000 centipoises.
From a cellulose article manufacturing process standpoint these differences are important because after dissolution the process steps (including cellulose recovery) are dependent on whether cellulose has entered into a covalent bond with the solubilizing reagent, i.e., has been derivatized. This is so in the case of the well-known and commercially practiced viscose process. When a cellulose derivative is processed into the shaped article, the derivative such as viscose is first partially coagulated in the extrusion bath and then subsequently hydrolyzed back to cellulose, i.e., cellulose is regenerated. During this hydrolysis and while the derivative is still in a "plastic" state, the reforming cellulose crystallites can be stretched and oriented to give desirable commercial properties such as high tensile strength or burst strength. However, a disadvantage of this general approach is that since a cellulose derivative has been hydrolyzed, additional byproducts are formed. This significantly complicates cellulose recovery.
By contrast in the nonderivative cellulose dissolution methods such as those using an NMMO/H.sub.2 O solvent solution, orienting the cellulose molecules during the reorganization of the cellulose article is more difficult because there is no covalent bond to break. So reorganization is essentially a physical dilution or decomplexation. However recovery is less complex and, at least in the cellulose/NMMO/H.sub.2 O system, commercially feasible.
The prior art such as McCorsley III U.S. Pat. No. 4,246,221 and East German Patent No. DD 218 121 has taught that such nonderivatized cellulose containing mixtures with NMMO and water may be forced through a nozzle and longitudinally guided through a 12 inch long air gap into a precipitating bath to form very small diameter solid fibers. More recently the nonderivatized cellulose fiber spinning prior art teaches that such long air path lengths should be avoided. As stated for example in Jurkovic et al U.S. Pat. No. 5,252,284, a long air gap leads to sticking of the fibers, uncertainties in spinning and fiber breakage at high degrees of drawing. According to Jurkovic et al, by using selected orifice diameters and nozzle channel lengths, the air gap is desirably reduced to at most 35 mm (1.4 inches).
It will be appreciated by those skilled in the art that manufacture of individual solid cellulose fibers by extrusion through orifices of 2-4 mils diameter is nonanalogous to manufacture of cellulose food casings which are extruded as a hollow tube of at least about 0.70 inches inside diameter with wall thickness typically on the order of 0.001 to about 0.004 inches.
In U.S. Pat. No. 5,277,857 ("the '857 Patent") there is disclosed a method of and apparatus for manufacturing cellulose food casing from a solution comprising nonderivatized cellulose, NMMO and water. The disclosure of the '857 Patent is incorporated herein by reference.
According to the '857 Patent, it was unexpectedly discovered that nonderivatized cellulose solutions are suitable for use in making cellulose food casings. The nonderivatized cellulose solution comprising cellulose, water and NMMO (hereinafter sometimes referred to as "dope") in a molten state can be extruded as a tubular film into a nonsolvent liquid such as a water bath.
For purposes of this specification "nonderivatized" cellulose means a cellulose which has not been subjected to covalent bonding with a solvent or reagent but which has been dissolved by association with a solvent or reagent through Van der Waals forces such as hydrogen bonding. "Nonsolvent" means a liquid which is not a cellulose solvent.
In the water bath, the NMMO solvent is extracted and the nonderivatized cellulose precipitates. The resulting gel tube can be treated with water, a polyhydric alcohol such as glycerine, or other water soluble softening agents such as a polyalkylene oxide or a polyalkylene glycol prior to drying.
The nonderivatized cellulose food casings prepared by the teachings of the "857 Patent are somewhat limited in the sense that their tensile strength properties are not equivalent to those of commercially employed viscose-derived cellulose casing. More particularly, based on a flat width of about 2.24 inches and wall thickness of about 0.80 mils, the machine direction (MD) tensile strength of the NMMO-based nonderivatized cellulose tube prepared according to the teachings of the '857 Patent is about 3.77 lbs/inch.mil, and the MD tensile strength of a viscose-derived NOJAX type cellulose food casing manufactured and sold by Viskase Corporation is about 4.18 lbs/inch-mil. So from the MD tensile strength standpoint, the two casings are comparable.
However, the transverse direction (TD) tensile strength of the NMMO-based nonderivatized cellulose tube is about 1.60 lbs/inch-mil, whereas a typical food casing made using the viscose process such as a NOJAX.RTM. casing made by Viskase Corporation has TD tensile strength of about 3.15 lbs/inch-mil. It will be apparent from the foregoing that the former's TD strength is limiting and further that the NMMO-based nonderivatized tube does not have balanced tensile strengths, i.e., the MD/TD is about 3.76 in contrast to the balanced NOJAX food casing tensile strength where the MD/TD ratio is about 1.33.
An object of this invention is to provide a method of forming a seamless cellulose tube (suitable for use as a food casing) from a solution comprising nonderivatized cellulose, tertiary amine N-oxide and water.
Another object is to provide such a method including extruding such a solution through an air gap and into a water bath so as to yield a cellulose tube with an improved TD tensile strength.
A further object is to provide such a method wherein improved tensile strength is gained by cooling the extruded solution in the air gap.